It is well known that dye images can be obtained by exposing in an imagewise manner a silver halide light-sensitive material, followed by a color development during which the reaction between an oxidized aromatic primary amine developer and a color-forming coupler takes place. In such a process, the subtractive color process is usually utilized for color reproduction, and cyan, magenta and yellow color images, which are complementary to red, green and blue colors, respectively, are formed. The reaction between a coupler and an oxidation product of a color developing agent proceeds at an active center of a coupler. A coupler having a hydrogen atom at its active center (i.e., 4-equivalent coupler) stoichiometrically requires as an oxidizing agent 4 moles of developable silver halide for forming 1 mole of dye through the coupling reaction. On the contrary, a coupler (or 2-equivalent coupler) having at its active center a group capable of being released in the form of an anion requires only 2 moles of developable silver halide for forming 1 mole of dye. Accordingly, by using a 2-equivalent coupler, the amount of silver halide to be used in a light-sensitive layer can be reduced and the layer per se can be made thinner, so that the time required for the processing of such a light-sensitive material can be shortened and dye images obtained therefrom can have an improved sharpness, compared with a light-sensitive material in which a 4-equivalent coupler is used. In addition, the coupling activity of a 2-equivalent coupler can be widely varied, depending on the property of the releasable group contained therein.
2-Equivalent couplers capable of releasing a group having a development-inhibiting effect are known and called development inhibitor releasing coupler (or DIR coupler). Such couplers are capable of inhibiting development in proportion to the quantity of developed silver and, therefore, can be effective for improving fineness, gradient and color reproducibility of the image. Couplers of this type can also be used in diffusion transfer processes, wherein their effects upon adjoining layers are utilized.
2-Equivalent couplers can also be provided with a releasable group containing a diffusible dye moiety. This class of couplers, which are referred to as diffusible dye-releasing coupler, can be utilized in a diffusion transfer process in which a dye image is formed from diffused dyes.
Certain colored 2-equivalent couplers can also be used to attain a masking effect necessary for color correction of dye images.
As mentioned hereinabove, 2-equivalent couplers can be provided with various functions, depending on the selection of releasable groups contained therein.
It is, therefore, an object of the present invention to provide a novel class of coupler with a new function.
On the other hand, regarding the recent trend in the development of silver halide light-sensitive materials, in particular, used for photographing, two major streams can be seen. One is the improvement in sensitivity, as typically shown by ASA 400 films, etc. The other is the improvement in image quality to cope with the miniaturization of film sizes. In connection with the former, investigations have been made on a variety of techniques, including, e.g., large size silver halide grains, couplers with higher activities, accelerated development, etc. However, the increase in sensitivity based on large size silver halide grains seems to be reaching its limit, as reported by G. C. Farnell and J. B. Chanter in J. Photogr. Sci., 9, 73 (1961). Accordingly, this technique is not expected to make much contribution in the future. In addition, the use of large size silver halide grains is accompanied by various disadvantages, such as deterioration in granularity. Couplers having higher sensitivities have also been studied extensively. Such couplers, however, have not made much contribution to sensitivities of silver halide light-sensitive materials, and proved to be disadvantageous for granularity. With regard to acceleration of development, various development accelerators, including hydrazine compounds, have hitherto been attempted in silver halide emulsion layers (mainly of black-and-white light-sensitive materials) or developing solutions therefor. However, in most cases, the incorporation of developing accelerators into emulsion layers or developing solutions is accompanied by such disadvantages as increase in fog and deterioration in granularity, and hence impractical.